[Windows] One TimeTicks clock: efficient/reliable high-res, with low-res fallback.

base/time/time_win.cc

Index: base/time/time_win.cc
diff --git a/base/time/time_win.cc b/base/time/time_win.cc
index 9b4f17d6613c7e60b8576adc9ae3b2a6afc8c8a3..a6109d1f1d5f82e887189b5213f09464f1a63766 100644
--- a/base/time/time_win.cc
+++ b/base/time/time_win.cc
@@ -307,13 +307,13 @@ DWORD timeGetTimeWrapper() {
   return timeGetTime();
 }
 
-DWORD (*tick_function)(void) = &timeGetTimeWrapper;
+DWORD (*g_tick_function)(void) = &timeGetTimeWrapper;
 
 // Accumulation of time lost due to rollover (in milliseconds).
-int64 rollover_ms = 0;
+int64 g_rollover_ms = 0;
 
 // The last timeGetTime value we saw, to detect rollover.
-DWORD last_seen_now = 0;
+DWORD g_last_seen_now = 0;
 
 // Lock protecting rollover_ms and last_seen_now.
 // Note: this is a global object, and we usually avoid these. However, the time
@@ -321,169 +321,160 @@ DWORD last_seen_now = 0;
 // easy to use a Singleton without even knowing it, and that may lead to many
 // gotchas). Its impact on startup time should be negligible due to low-level
 // nature of time code.
-base::Lock rollover_lock;
+base::Lock g_rollover_lock;
 
 // We use timeGetTime() to implement TimeTicks::Now().  This can be problematic
 // because it returns the number of milliseconds since Windows has started,
 // which will roll over the 32-bit value every ~49 days.  We try to track
 // rollover ourselves, which works if TimeTicks::Now() is called at least every
 // 49 days.
-TimeDelta RolloverProtectedNow() {
-  base::AutoLock locked(rollover_lock);
+TimeTicks RolloverProtectedNow() {
+  base::AutoLock locked(g_rollover_lock);
   // We should hold the lock while calling tick_function to make sure that
   // we keep last_seen_now stay correctly in sync.
-  DWORD now = tick_function();
-  if (now < last_seen_now)
-    rollover_ms += 0x100000000I64;  // ~49.7 days.
-  last_seen_now = now;
-  return TimeDelta::FromMilliseconds(now + rollover_ms);
+  DWORD now = g_tick_function();
+  if (now < g_last_seen_now)
+    g_rollover_ms += 0x100000000I64;  // ~49.7 days.
+  g_last_seen_now = now;
+  return TimeTicks() + TimeDelta::FromMilliseconds(now + g_rollover_ms);
 }
 
-bool IsBuggyAthlon(const base::CPU& cpu) {
-  // On Athlon X2 CPUs (e.g. model 15) QueryPerformanceCounter is
-  // unreliable.  Fallback to low-res clock.
-  return cpu.vendor_name() == "AuthenticAMD" && cpu.family() == 15;
-}
-
-// Overview of time counters:
+// Discussion of tick counter options on Windows:
+//
 // (1) CPU cycle counter. (Retrieved via RDTSC)
 // The CPU counter provides the highest resolution time stamp and is the least
-// expensive to retrieve. However, the CPU counter is unreliable and should not
-// be used in production. Its biggest issue is that it is per processor and it
-// is not synchronized between processors. Also, on some computers, the counters
-// will change frequency due to thermal and power changes, and stop in some
-// states.
+// expensive to retrieve. However, on older CPUs, two issues can affect its
+// reliability: First it is maintained per processor and not synchronized
+// between processors. Also, the counters will change frequency due to thermal
+// and power changes, and stop in some states.
 //
 // (2) QueryPerformanceCounter (QPC). The QPC counter provides a high-
-// resolution (100 nanoseconds) time stamp but is comparatively more expensive
-// to retrieve. What QueryPerformanceCounter actually does is up to the HAL.
-// (with some help from ACPI).
-// According to http://blogs.msdn.com/oldnewthing/archive/2005/09/02/459952.aspx
-// in the worst case, it gets the counter from the rollover interrupt on the
+// resolution (<1 microsecond) time stamp. On most hardware running today, it
+// auto-detects and uses the constant-rate RDTSC counter to provide extremely
+// efficient and reliable time stamps.
+//
+// On older CPUs where RDTSC is unreliable, it falls back to using more
+// expensive (20X to 40X more costly) alternate clocks, such as HPET or the ACPI
+// PM timer, and can involve system calls; and all this is up to the HAL (with
+// some help from ACPI). According to
+// http://blogs.msdn.com/oldnewthing/archive/2005/09/02/459952.aspx, in the
+// worst case, it gets the counter from the rollover interrupt on the
 // programmable interrupt timer. In best cases, the HAL may conclude that the
 // RDTSC counter runs at a constant frequency, then it uses that instead. On
 // multiprocessor machines, it will try to verify the values returned from
 // RDTSC on each processor are consistent with each other, and apply a handful
 // of workarounds for known buggy hardware. In other words, QPC is supposed to
-// give consistent result on a multiprocessor computer, but it is unreliable in
-// reality due to bugs in BIOS or HAL on some, especially old computers.
-// With recent updates on HAL and newer BIOS, QPC is getting more reliable but
-// it should be used with caution.
+// give consistent results on a multiprocessor computer, but for older CPUs it
+// can be unreliable due bugs in BIOS or HAL.
 //
-// (3) System time. The system time provides a low-resolution (typically 10ms
-// to 55 milliseconds) time stamp but is comparatively less expensive to
-// retrieve and more reliable.
-class HighResNowSingleton {
- public:
-  HighResNowSingleton()
-      : ticks_per_second_(0),
-        skew_(0) {
-
-    base::CPU cpu;
-    if (IsBuggyAthlon(cpu))
-      return;
-
-    // Synchronize the QPC clock with GetSystemTimeAsFileTime.
-    LARGE_INTEGER ticks_per_sec = {0};
-    if (!QueryPerformanceFrequency(&ticks_per_sec))
-      return; // QPC is not available.
-    ticks_per_second_ = ticks_per_sec.QuadPart;
-
-    skew_ = UnreliableNow() - ReliableNow();
-  }
-
-  bool IsUsingHighResClock() {
-    return ticks_per_second_ != 0;
-  }
-
-  TimeDelta Now() {
-    if (IsUsingHighResClock())
-      return TimeDelta::FromMicroseconds(UnreliableNow());
-
-    // Just fallback to the slower clock.
-    return RolloverProtectedNow();
+// (3) System time. The system time provides a low-resolution (from ~1 to ~15.6
+// milliseconds) time stamp but is comparatively less expensive to retrieve and
+// more reliable. Time::EnableHighResolutionTimer() and
+// Time::ActivateHighResolutionTimer() can be called to alter the resolution of
+// this timer; and also other Windows applications can alter it, affecting this
+// one.
+
+using NowFunction = TimeTicks (*)(void);
+
+TimeTicks InitialNowFunction();
+TimeTicks InitialSystemTraceNowFunction();
+
+// See "threading notes" in InitializeNowFunctionPointers() for details on how
+// concurrent reads/writes to these globals has been made safe.
+NowFunction g_now_function = &InitialNowFunction;
+NowFunction g_system_trace_now_function = &InitialSystemTraceNowFunction;
+int64 g_qpc_ticks_per_second = 0;
+
+// As of January 2015, use of <atomic> is forbidden in Chromium code. This is
+// what std::atomic_thread_fence does on Windows on all Intel architectures:

[brucedawson, 2015/01/08 19:37:48]

To be clear, this is what atomic_thread_fence does if memory_order is not
memory_order_seq_cst. If memory_order is memory_order_seq_cst then a full fence
instruction is required. We don't need that so this is just a comment error.

[miu, 2015/01/08 19:45:56]

Done.

+#define atomic_thread_fence(memory_order) _ReadWriteBarrier();

[jamesr, 2015/01/08 20:31:54]

this doesn't follow macro naming rules:
http://google-styleguide.googlecode.com/svn/trunk/cppguide.html#Macro_Names

naming it as if it were a standard feature and not having it actually do what
the standard says is a pretty bad idea.  If you want a read write barrier then
just say so.  we don't have any intention of supporting windows on non-intel
platforms.

[miu, 2015/01/08 22:16:00]

Done.  Fixed capitalization.
That's not the case here.  The standard only specifies the behavior
requirements, but it's up to the implementation as to how that behavior is
achieved.  The point of using the macro is to communicate the specific memory
ordering behavior needed at the two codepoints.  If I put _ReadWriteBarrier() at
the two codepoints instead, this forces people to reverse-engineer/guess my
original intentions.

It feels really silly/stupid not to use <atomic> here.  The rationale for the
ban (http://chromium-cpp.appspot.com) doesn't apply to what I'm doing here. 
This is a platform-specific module, where the only toolchains that would touch
this code DO support <atomic>.

+
+TimeDelta QPCValueToTimeDelta(LONGLONG qpc_value) {
+  // Ensure that the assignment to |g_qpc_ticks_per_second|, made in
+  // InitializeNowFunctionPointers(), has happened by this point.
+  atomic_thread_fence(memory_order_acquire);
+
+  DCHECK_GT(g_qpc_ticks_per_second, 0);
+
+  // If the QPC Value is below the overflow threshold, we proceed with
+  // simple multiply and divide.
+  if (qpc_value < Time::kQPCOverflowThreshold) {
+    return TimeDelta::FromMicroseconds(
+        qpc_value * Time::kMicrosecondsPerSecond / g_qpc_ticks_per_second);
   }
+  // Otherwise, calculate microseconds in a round about manner to avoid
+  // overflow and precision issues.
+  int64 whole_seconds = qpc_value / g_qpc_ticks_per_second;
+  int64 leftover_ticks = qpc_value - (whole_seconds * g_qpc_ticks_per_second);
+  return TimeDelta::FromMicroseconds(
+      (whole_seconds * Time::kMicrosecondsPerSecond) +
+      ((leftover_ticks * Time::kMicrosecondsPerSecond) /
+       g_qpc_ticks_per_second));
+}
 
-  int64 GetQPCDriftMicroseconds() {
-    if (!IsUsingHighResClock())
-      return 0;
-    return abs((UnreliableNow() - ReliableNow()) - skew_);
-  }
-
-  int64 QPCValueToMicroseconds(LONGLONG qpc_value) {
-    if (!ticks_per_second_)
-      return 0;
-    // If the QPC Value is below the overflow threshold, we proceed with
-    // simple multiply and divide.
-    if (qpc_value < Time::kQPCOverflowThreshold)
-      return qpc_value * Time::kMicrosecondsPerSecond / ticks_per_second_;
-    // Otherwise, calculate microseconds in a round about manner to avoid
-    // overflow and precision issues.
-    int64 whole_seconds = qpc_value / ticks_per_second_;
-    int64 leftover_ticks = qpc_value - (whole_seconds * ticks_per_second_);
-    int64 microseconds = (whole_seconds * Time::kMicrosecondsPerSecond) +
-                         ((leftover_ticks * Time::kMicrosecondsPerSecond) /
-                          ticks_per_second_);
-    return microseconds;
-  }
-
- private:
-  // Get the number of microseconds since boot in an unreliable fashion.
-  int64 UnreliableNow() {
-    LARGE_INTEGER now;
-    QueryPerformanceCounter(&now);
-    return QPCValueToMicroseconds(now.QuadPart);
-  }
-
-  // Get the number of microseconds since boot in a reliable fashion.
-  int64 ReliableNow() {
-    return RolloverProtectedNow().InMicroseconds();
-  }
-
-  int64 ticks_per_second_;  // 0 indicates QPF failed and we're broken.
-  int64 skew_;  // Skew between lo-res and hi-res clocks (for debugging).
-};
-
-static base::LazyInstance<HighResNowSingleton>::Leaky
-    leaky_high_res_now_singleton = LAZY_INSTANCE_INITIALIZER;
-
-HighResNowSingleton* GetHighResNowSingleton() {
-  return leaky_high_res_now_singleton.Pointer();
+TimeTicks QPCNow() {
+  LARGE_INTEGER now;
+  QueryPerformanceCounter(&now);
+  return TimeTicks() + QPCValueToTimeDelta(now.QuadPart);
 }
 
-TimeDelta HighResNowWrapper() {
-  return GetHighResNowSingleton()->Now();
+bool IsBuggyAthlon(const base::CPU& cpu) {
+  // On Athlon X2 CPUs (e.g. model 15) QueryPerformanceCounter is unreliable.
+  return cpu.vendor_name() == "AuthenticAMD" && cpu.family() == 15;
 }
 
-typedef TimeDelta (*NowFunction)(void);
+void InitializeNowFunctionPointers() {
+  LARGE_INTEGER ticks_per_sec = {0};
+  if (!QueryPerformanceFrequency(&ticks_per_sec))
+    ticks_per_sec.QuadPart = 0;
 
-bool CPUReliablySupportsHighResTime() {
+  // If Windows cannot provide a QPC implementation, both Now() and
+  // NowFromSystemTraceTime() must use the low-resolution clock.
+  //
+  // If the QPC implementation is expensive and/or unreliable, Now() will use
+  // the low-resolution clock, but NowFromSystemTraceTime() will use the QPC (in
+  // the hope that it is still useful for tracing purposes). A CPU lacking a
+  // non-stop time counter will cause Windows to provide an alternate QPC
+  // implementation that works, but is expensive to use. Certain Athlon CPUs are
+  // known to make the QPC implementation unreliable.
+  //
+  // Otherwise, both Now functions can use the high-resolution QPC clock. As of
+  // 4 January 2015, ~68% of users fall within this category.
+  NowFunction now_function;
+  NowFunction system_trace_now_function;
   base::CPU cpu;
-  if (!cpu.has_non_stop_time_stamp_counter() ||
-      !GetHighResNowSingleton()->IsUsingHighResClock())
-    return false;
-
-  if (IsBuggyAthlon(cpu))
-    return false;
+  if (ticks_per_sec.QuadPart <= 0) {
+    now_function = system_trace_now_function = &RolloverProtectedNow;
+  } else if (!cpu.has_non_stop_time_stamp_counter() || IsBuggyAthlon(cpu)) {
+    now_function = &RolloverProtectedNow;
+    system_trace_now_function = &QPCNow;
+  } else {
+    now_function = system_trace_now_function = &QPCNow;
+  }
 
-  return true;
+  // Threading note 1: In an unlikely race condition, it's possible for two or
+  // more threads to enter InitializeNowFunctionPointers() in parallel. This is
+  // not a problem since all threads should end up writing out the same values
+  // to the global variables.
+  //
+  // Threading note 2: A release fence is placed here to ensure, from the
+  // perspective of other threads using the function pointers, that the
+  // assignment to |g_qpc_ticks_per_second| happens before the function pointers
+  // are changed.
+  g_qpc_ticks_per_second = ticks_per_sec.QuadPart;
+  atomic_thread_fence(memory_order_release);
+  g_now_function = now_function;
+  g_system_trace_now_function = system_trace_now_function;
 }
 
-TimeDelta InitialNowFunction();
-
-volatile NowFunction now_function = InitialNowFunction;
+TimeTicks InitialNowFunction() {
+  InitializeNowFunctionPointers();
+  return g_now_function();
+}
 
-TimeDelta InitialNowFunction() {
-  if (!CPUReliablySupportsHighResTime()) {
-    InterlockedExchangePointer(
-        reinterpret_cast<void* volatile*>(&now_function),
-        &RolloverProtectedNow);
-    return RolloverProtectedNow();
-  }
-  InterlockedExchangePointer(
-        reinterpret_cast<void* volatile*>(&now_function),
-        &HighResNowWrapper);
-  return HighResNowWrapper();
+TimeTicks InitialSystemTraceNowFunction() {
+  InitializeNowFunctionPointers();
+  return g_system_trace_now_function();
 }
 
 }  // namespace
@@ -491,27 +482,24 @@ TimeDelta InitialNowFunction() {
 // static
 TimeTicks::TickFunctionType TimeTicks::SetMockTickFunction(
     TickFunctionType ticker) {
-  base::AutoLock locked(rollover_lock);
-  TickFunctionType old = tick_function;
-  tick_function = ticker;
-  rollover_ms = 0;
-  last_seen_now = 0;
+  base::AutoLock locked(g_rollover_lock);
+  TickFunctionType old = g_tick_function;
+  g_tick_function = ticker;
+  g_rollover_ms = 0;
+  g_last_seen_now = 0;
   return old;
 }
 
 // static
 TimeTicks TimeTicks::Now() {
-  return TimeTicks() + now_function();
+  return g_now_function();
 }
 
 // static
-TimeTicks TimeTicks::HighResNow() {
-  return TimeTicks() + HighResNowWrapper();
-}
-
-// static
-bool TimeTicks::IsHighResNowFastAndReliable() {
-  return CPUReliablySupportsHighResTime();
+bool TimeTicks::IsHighResolution() {
+  if (g_now_function == &InitialNowFunction)
+    InitializeNowFunctionPointers();
+  return g_now_function == &QPCNow;
 }
 
 // static
@@ -522,27 +510,17 @@ TimeTicks TimeTicks::ThreadNow() {
 
 // static
 TimeTicks TimeTicks::NowFromSystemTraceTime() {
-  return HighResNow();
-}
-
-// static
-int64 TimeTicks::GetQPCDriftMicroseconds() {
-  return GetHighResNowSingleton()->GetQPCDriftMicroseconds();
+  return g_system_trace_now_function();
 }
 
 // static
 TimeTicks TimeTicks::FromQPCValue(LONGLONG qpc_value) {
-  return TimeTicks(GetHighResNowSingleton()->QPCValueToMicroseconds(qpc_value));
-}
-
-// static
-bool TimeTicks::IsHighResClockWorking() {
-  return GetHighResNowSingleton()->IsUsingHighResClock();
+  return TimeTicks() + QPCValueToTimeDelta(qpc_value);
 }
 
 // TimeDelta ------------------------------------------------------------------
 
 // static
 TimeDelta TimeDelta::FromQPCValue(LONGLONG qpc_value) {
-  return TimeDelta(GetHighResNowSingleton()->QPCValueToMicroseconds(qpc_value));
+  return QPCValueToTimeDelta(qpc_value);
 }